A dampening unit for use with an offset printing press is known from German Published, Examined Patent application DE 1 611 313. A dampening agent is atomized in a pulse-like manner, and at a selectable pulse length as a function of the number of revolutions of a forme cylinder. This atomized dampening agent is intermittently applied to the surface of a roller of the dampening unit by the use of nozzles. German Published, Examined Patent Application DE 1 761 313 complements DE 1 611 313 to the extent that a pulse length and a pulse sequence frequency can be adjusted. The pulse length is greater at a low printing speed and is shorter at a high printing speed. Alternatively, the number of spray pulses emitted per revolution of the forme cylinder is higher at a low printing speed and is lower at a higher printing speed.
A spray dampening unit of a printing press is known from U.S. Pat. No. 2,231,694. The nozzles eject a dampening agent in an adjustable amount at predetermined chronological intervals onto a dampening roller.
A spray dampening unit of a printing press is known from U.S. Pat. No. 5,038,681. A dampening agent can be applied, by the use of nozzles, to the surface of a roller of the spray dampening unit at a fixed pulse length, but with a variable pulse sequence spacing, which spacing is selected as a function of the number of revolutions of a forme cylinder.
A spray dampening unit of a printing press is known from DE 100 05 908 A1. A surface, preferably of a rotating roller, is sprayed with a dampening agent by a plurality of spray nozzles. The spray nozzles are each activated with a predetermined frequency and phase shift. Thus, the spray nozzles spray sequentially and cyclically in a fixed order, wherein the length of time between the activation of the same spray nozzle is always the same. The pulse length, i.e. the time during which the spray nozzle is open, is also preferably the same for all of the spray nozzles. The circumferential length of the area sprayed on the surface of the roller, and a circumferential spacing between sequential sprayed areas are a function of the work cycle of the spray nozzles and of a surface speed of the roller. However, no discussion is found in DE 100 05 908 A1 as to what conditions must be maintained between the work cycle of the spray nozzles, or the surface speed of the roller, and a duration of the revolution of a forme cylinder in order to achieve as uniform as possible an application of the spray agent to the forme cylinder at a contact point between the roller and the forme cylinder.
A spray dampening unit of a printing press is known from U.S. Pat. No. 4,649,818. An electronic control circuit controls spray nozzles as a function of a detected press speed of the printing press. A frequency of the spraying pulses emitted by the spray nozzles preferably has a non-linear connection with the speed of the press. It is provided, particularly in case of a fault in the electronic control circuit, to set the spraying frequency manually, such as, for example, with the use of graphic aids representing a connection between the speed of the press and a spraying frequency to be set. There is also no suggestion in U.S. Pat. No. 4,649,818 whether, and if so, which condition between the work cycle of the spray nozzles, or the surface speed of a dampening unit roller, must be maintained to achieve as uniform as possible an application of the spray agent to the forme cylinder at a contact point between the dampening unit roller and the forme cylinder.
Spray dampening units, which intermittently release a dampening agent, such as, for example, a water aerosol, through spray nozzles, and which dampening agent wets a rotating roller with moisture, have been employed for years in offset printing presses. This thin water film is transferred, via a further roller or rollers of the spray dampening unit to a printing forme on the forme cylinder. The sprayed roller and subsequent transfer rollers rotate synchronously with the speed of the press as determined by the number of revolutions of the forme cylinder.
A printing process typically requires different amounts of moisture, depending on the speed of the press and the print pattern. The relationship between the speed of the press and the required amount of moisture can be taken from a so-called dampening curve, which dampening curve is a graphic representation of a dampening degree D as a function of the number of revolutions of the forme cylinder. Thus, the dampening curve indicates what dampening degree D is to be set in a dampening agent dispenser, such as, for example, a nozzle in a spray crosspiece. The dampening degree D marks a ratio between a dampening agent throughput to be set at a dampening agent dispenser and a maximum dampening agent throughput.Dampening degree D=tON/tON+tOFF wherein tON=the length of time of the dampening agent throughput and tOFF=the length of time of the dampening agent blockage of stoppage.
In addition to the dampening fluid requirement set by the dampening curve, the amount of moisture can be varied by an operator of the printing press and can be set to any arbitrary value within a value ranging between a total blockage or stoppage of the spray nozzles up to their maximum amount of flow-through. In this case, a change in the amount of moisture emitted by the spray nozzle is achieved by the use of the ratio between their spray time Ton and off-time Toff. Actual operations are preferably performed with as constant as possible an “on” time, so that only the “off” time is varied. Thus, the scanning time ratio, or on-time to off-time changes, together with the requirement for an amount of moisture, as well as the spraying frequency f=1/(Ton+Toff). When selecting the spraying on time Ton, it should be noted that a spray nozzle requires a definite minimum amount of time for forming its spray cone, as well as for the emergence of a defined amount of moisture, so that the spray time Ton can therefore not be set arbitrarily low.
Because of the intermittent manner of the spraying of a dampening agent on a surface area of a rotating roller, a serious disadvantage arises. An uneven, and therefore an undesirable overlapping of the sprayed-on dampening agent can arise as a function of the rotating frequency of the sprayed roller and as a function of the spraying frequency of the nozzle onto the sprayed roller. As a result, such an undesirable overlapping of the dampening agent also occurs on the surface area of the forme cylinder if, in case of an unfavorable correlation between the rotating frequency of the roller and the spraying frequency of the nozzle, the same, or at least a part of the same area on the circumference of the roller is sprayed again and again during each revolution of the roller. In the end, too much dampening agent is applied to some areas on the surface of the cylinder, and too little dampening agent is applied to other areas. The rotating frequency of the roller, and the spraying frequency of the nozzle then reach a state which is called beating interference in oscillation technology. An uneven distribution of the dampening agent has extremely negative effects when imprinting a material, because it leads to considerable ink variations on the material to be imprinted. The danger of the occurrence of such a beating interference is considerable, if no appropriate countermeasures are taken, since the number of revolutions of the printing press, as well as the amount of moisture, can be freely selected by the operator. Thus, this undesirable beating interference effect can occur at any arbitrary operational state.
This beating interference effect arises analogously if more than one nozzle is arranged over the length of the roller. In accordance with the above description, the individual nozzles are separately controlled, and exactly the same effect can occur between two adjoining nozzles. Adjoining nozzles may spray at different frequencies because of a different requirement of the amount of moisture existing over the length of the roller, so that a beating interference between the nozzles occurs, and therefore a very uneven application of dampening agent to the roller is the result.